With the ongoing development in electronics technology, more and more products used in the home and at work are electronically based. Televisions, VCRs, computers and their monitors, printers, copiers, etc., are common electronic devices, and such devices require a source of power for operation. However, electronic devices are not able to directly use the 110V or 220V provided by electric power companies, and said electronic devices are installed with semiconductors, etc., that are driven by roughly 5V to 10V. A power supply performs the operation of transforming the 110V or 220V source to a suitable level of voltage, e.g., 5V to 10V.
The switched-mode power supply is used in many electronic products since it is able to supply a stable power. A conventional switched-mode power supply is shown in FIGS. 1–4. Referring to FIG. 1, the switched-mode power supply, which receives an input power (Vin) and provides an output power (Vo) according to a load includes, a first power supply unit (100) that receives the input power (Vin) and undergoes switching according to variations in the output power (Vo) to thereby supply a power, the variations in the output power (Vo) occurring according to a load; and an output power supply unit (200) that receives the power output by said first power supply unit (100) through a coil winding ratio and generates an output power (Vo) for driving a load.
The first power supply unit (100) includes a first power converter (110) for receiving the input power (Vin), rectifying the same, then outputting a resulting power; an output power sensing unit (120) for sensing the output power (Vo) that is output to the load from said output power supply unit (200); a switching controller (150), which outputs a signal for controlling the timing of the switching of said output power according to a signal output from the output power sensing unit (120); and a switching transistor (MOS130) that switches On and Off to transmit the power converted by the first power converter (110) to the output power supply unit (200).
The first power converter (110) includes a bridge circuit (BR110) that receives the input power (Vin), performs a wave rectification of said input power, then outputs a resulting power; a first resistor (R111), with a first terminal connected to an output terminal of said bridge circuit (BR110); a capacitor (C110), with a first terminal connected to a second terminal of said resistor (R111), and a second terminal which is grounded; a first coil (L110), with a first terminal connected to the second terminal of said resistor (R111), and a second terminal connected to a drain of switching transistor (MOS130); and a second resistor (R112), with a first terminal connected to the output terminal of said bridge circuit (BR110).
The output power sensing unit (120) includes, a phototransistor (PT120) for receiving, through a base thereof, an output power that is sensed and undergoes feedback from the output power supply unit (200), said phototransistor (PT120) receiving the output power as a light signal; a first capacitor (C120), with a first terminal connected to a collector of said phototransistor (PT120), and a second terminal which is grounded; a diode (D120), with a cathode connected to the collector of said phototransistor (PT120); a power sensing coil (L120), with a first terminal connected to an anode of said diode (D120), and a second terminal which is grounded, the power sensing coil (L120) being connected in parallel to the first coil (L110) of the first power converter (110); a first resistor (R120), with a first terminal connected to an emitter of said phototransistor (PT120); a second capacitor (C121), with a first terminal connected to the second terminal of said first resistor (R120), and a second terminal which is grounded; a second resistor (R121), with a first terminal connected to the second terminal of said first resistor (R120), and a second terminal connected to the source of said switching transistor (MOS130); and a third resistor (R122), with a first terminal connected to the source of said switching transistor (MOS130), and a second terminal which is grounded.
The switching controller (150) includes a comparator (COM150), with a non-inverting terminal which receives a sensed signal from the output power sensing unit (120), and an inverting terminal which receives a first reference voltage of a predetermined value; a flip-flop (SR150), with a reset terminal connected to an output terminal of said comparator (COM150), and a set terminal which is connected to a clock signal; an OR gate (OR150), with a first input terminal connected to an inverted output terminal of the flip-flop (SR150), and a second input terminal connected to the clock signal; a first transistor (Q151), with a collector connected to a drive power source (Vcc), a base connected to an inverted output terminal of the OR gate (OR150), and an emitter connected to a gate of the switching transistor (MOS130); and a second transistor (Q152), with a base connected to a non-inverted output terminal of the OR gate (OR150), and a collector connected to the gate of the switching transistor (MOS130).
The output power supply unit (200) includes an output power generating unit (210) that receives the power output from the first power supply unit (100) and generates a power for driving a load; and an output power feedback unit (220) for sensing the power generated by an output from the output power generating unit (210) and performing feedback of the power to the first power supply unit (100).
The output power generating unit (210) of the output power supply unit (200) includes a second coil (L210) for receiving the power applied from the first power supply unit (100) through an induced current; a diode (D210), with an anode connected to a first terminal of the second coil (L210) (a second terminal of the second coil (L210) being grounded); and a capacitor (C210), with a first terminal connected to a cathode of the diode (D210), and a second terminal which is grounded.
The output power feedback unit (220) of the output power supply unit (200) includes a first resistor (R220), with a first terminal which receives the output power of the output power generating unit (210); a second resistor (R230), with a first terminal connected to a second terminal of said first resistor (R220), and a second terminal which is grounded; an operational amplifier (OP260), with an inverting terminal connected to the second terminal of said first resistor (R220), and a non-inverting terminal which receives a reference voltage; a transistor (Q270), with a base connected to an output terminal of the operational amplifier (OP260), and an emitter which is grounded; a capacitor (C250), with a first terminal connected to the second terminal of said first resistor (R220); a fourth resistor (R250), with a first terminal connected to a second terminal of said capacitor (C250), and a second terminal connected to a collector of said transistor (Q270); a third resistor (R240), with a first terminal which receives an output power of the output power generating unit (210); and a photodiode (PD240), with an anode connected to a second terminal of said third resistor (R240), a cathode connected to the collector of said transistor (Q270) to emit a light proportional to an amount of a current passing through the photodiode (PD240) for output to the first power supply unit (100).
The conventional switched-mode power supply structure operated in the following manner. The first power supply unit (100) typically receives the input power (Vin) and generates a power of a suitable level. This power is output through the first coil (L110) by the switching operation of the switching transistor (MOS130).
The output power supply unit (200) receives the power by induction through the second coil (L210), which opposes the first coil (L110), then provides a power required to drive various electronic devices. The power applied from the first power supply unit (100) and the power supplied to the load are adjusted by the charging and discharging of the capacitor (C210).
In order to control the On and Off operation of the switching transistor (MOS130), which regulates the power applied to the output power supply unit (200), the first power supply unit (100) senses the level of the output power (Vo) that is supplied to the load from the output power supply unit (200), and receives feedback of the output power (Vo). The sensing operation is performed in the output power feedback unit (220), and the transmission of the feedback signal is performed by a photo coupler, which is realized by the photodiode (PD240) and the phototransistor (PT120) of the first power supply unit (100).
That is, part of the output power (Vo) inverted in the capacitor (C210) of the output power generating unit (210) is detected by a resistance ratio between the first resistor (R220) and the second resistor (R230) of the feedback unit (220), then this value is compared in the operational amplifier (OP260) with a pre-installed reference voltage (Vref). Accordingly, a current (Iphoto) flowing to the photodiode (PD240) is determined. The photodiode (PD240) then emits a corresponding amount of light, which is sensed in the phototransistor (PT120) of the output power sensing unit (120) by passing through the base of the phototransistor (PT120).
As a result, a current (Ipt) of an amount corresponding to the amount of light sensed by the phototransistor (PT120) is generated, after which the current (Ipt) flows to the second resistor (R121) through the first resistor (R120). A resulting voltage drop VR121 across the second resistor (R121) is combined with a voltage VR122 of the third resistor (R122) such that an output value of the comparator (COM150) varies according to the voltage drop VR121 across the second resistor (R121).
Further, the second resistor (R121) and the second capacitor (C121) turn on the switching transistor (MOS130) to thereby operate as a filter circuit for preventing noise occurring as a result of a surge current. That is, if the output value of the comparator (COM150) is a logical HIGH value, the flip-flop (SR150) is reset such that the output value of the non-inverting terminal of the OR gate (OR150) becomes LOW and the output value of the inverting terminal of the OR gate (OR150) becomes HIGH.
The value of the output signal of said OR gate (OR150) is determined by an output signal of a clock generator (OSC) as shown in A of FIG. 2, and by the value of the inverting output terminal of the flip-flop (SR150) as shown in D of FIG. 2. Further, the value of the inverting output terminal of said flip-flop (SR150) is determined by the output signal of the comparator (COM150) as shown in C of FIG. 2, and the output signal of said comparator (COM150) is determined by a current (Isense) input to the non-inverting input terminal of the comparator (COM150) as shown in B of FIG. 2.
The value of the inverting output terminal of said OR gate (OR150), with input to the base of the first transistor (Q151), is opposite the output signal of the inverting output terminal of said flip-flop (SR150). In addition, if the second transistor (Q152) is turned On, the switching transistor (MOS130) is turned Off such that power transmission does not occur. However, if said first transistor (Q151) is turned On, the said switching transistor (MOS130) is turned On such that a first power is transmitted to the output power supply unit (200).
Therefore, in summary, if the output power (Vo) is increased, the amount of the current (Iphoto)flowing to the photodiode (PD240) is increased, and at the same time, the current (Ipt) flowing to the phototransistor (PT120) is also increased. Accordingly, the voltage drop across the second resistor (R121) is increased, and a DC offset voltage rises, resulting in a reduction in the On time of the switching transistor (MOS130).
If the On time of the switching transistor (MOS130) is reduced, the power transmitted to the output power supply unit (200) is reduced, and the time that the capacitor (C210) of the output power generating unit (210) is charged is reduced. This ultimately results in a reduction of the output power (Vo).
As described above, the conventional switched-mode power supply detects the output power (Vo), and adjusts the switching time of the switching transistor (MOS130) according to this value to thereby vary the output power (Vo). Typically, the conventional switched-mode power supply operating as described above is used to provide a stable supply of power to televisions, computer monitors, VCRs, etc. Such electronic devices often use a remote control to provide convenience to the user. With the provision of this capability, a minimum of transmitting and receiving circuitry is provided to enable control signals of the remote control to be received even when the controlled device is not being used. That is, a stand-by mode is supported in these devices with remote-control capability.
However, with the conventional switched-mode power supply operating as described above, even in a cut-off mode or stand-by mode, in which the power load is low as when the electronic device is not operating and only a minimal amount of stand-by power is required, the electronic device comes to operate in a normal mode identical to when the device is normally operating, and the power loss occurring as a result of the switching operation of the switching transistor (MOS130) is greater than when power is supplied to a load.
That is, there is always a loss of power in the conventional switched-mode power supply because of the switching operation of the switching transistor (MOS130). In the case where an electronic device operates normally such that the load is large, the switching loss is not as large as the power transmitted to the electronic device. However, if the electronic device operating as a load is in a cut-off mode or a stand-by mode, although there is not a large demand of power for operation since only a minimum amount of circuitry required to maintain such a stand-by state is operated, a great deal of power loss nevertheless occurs by the switching operation of the switching transistor (MOS130).
Further, the amount of time that the switching transistor (MOS130) is controlled to On decreases in tandem with decreases in the size of the load. If the size of the load is extremely small, the amount of time the switching transistor (MOS130) is controlled to On also decreases significantly. If this short On time of the switching transistor (MOS130) is unable to be realized by the circuit, suitable power supply control is not possible.